1. Field of the Invention
The invention relates to a wafer pedestal for supporting a wafer in a semiconductor wafer processing system and, more particularly, to a wafer pedestal with a purge ring that directs purge gas to the periphery of the pedestal.
2. Description of the Background Art
Titanium nitride (TiN) film is widely used as a barrier or contact layer in integrated circuit fabrication, particularly for device applications. A TiN film may be formed by metal organic chemical vapor deposition (MOCVD) using precursors such as tetrakis (dialkylamino)-titanium, or Ti(NR.sub.2)4, where R is an alkyl group. For example, U.S. Pat. No. 5,246,881, issued on Sep. 21, 1993, discloses thermal decomposition of tetrakis (dimethylamino)-titanium, or TDMAT, in combination with an activated species, for TiN deposition at temperatures of 200-600.degree. C. and pressures of about 0.1 to 100 torr. Another U.S. Pat. No. 5,576,071 ('071 patent), issued on Nov. 19, 1996, discloses a similar TiN deposition process in the presence of a reactive carrier gas such as nitrogen at a pressure of 0.1-10 torr and a temperature in the range of 200-700.degree. C.
While the choice of deposition parameters is primarily dictated by the desired characteristics in the deposited film, it is also constrained by the need for compatibility with other materials already present on the wafer substrate. For example, for 0.25 .mu.m device applications, the capacitance of metal interconnects can contribute significantly to signal delays. To satisfy both speed and cross-talk requirements between metal interconnects, it is preferable that insulators be made of materials having a low dielectric constant (i.e., low K dielectric materials having K less than 3.8). However, current low K dielectric materials, which include a wide variety of fluorinated organic or inorganic compounds, are stable only up to about 400.degree. C. Thus, backend processes including TiN deposition should preferably be performed at relatively low temperatures to ensure compatibility with these low K dielectrics and avoid adverse effects in device characteristics.
Aside from judicious choice and control of process parameters, a viable production-worthy process also needs to control particulate contamination, minimize equipment downtime, and facilitate ease of maintenance, among others. For deposition processes such as titanium nitride film deposition, it is inevitable that some deposits will be formed over interior surfaces of the process chamber, or other components inside the chamber. These deposits may, during the course of wafer processing or transport, flake off and land onto the wafers. Such contamination will adversely affect both the yield and reliability of the fabricated semiconductor devices. When a wafer heater is biased during post deposition plasma annealing, a film deposit formed on the heater surface leads to micro-arcing from the film deposit to, for example, the chamber wall. Such arcing introduces contaminants into the chamber environment.
Therefore, a need exists in the art for a low temperature TiN film deposition apparatus which minimizes undesirable deposits with improved control of particulate contamination and ease of equipment maintenance.